Is it possible to animate hair particle (object) size?
I'm getting the same result as you, changing the value after keyframing the Size property reverts to the previous keyframed value; it may be a bug or a limitation of the current system.

As a workaround, you can animate the value through the Graph Editor.

Just keyframe it regularly as you already did, then open the Graph Editor, select the object and move the curve vertex corresponding to the desired keyframe up or down according to the effect you wish to achieve.